Atomic frequency standards generate and maintain a standard frequency output by using the transition between two well-defined energy levels of an atom and the associated constant frequency to control the frequency of a frequency generating means. The atomic transition between two energy levels is employed as a highly stable frequency reference to which the frequency of a variable frequency oscillator, such as a voltage controlled oscillator (VCO) can be electronically locked. The high stability and relative insensitivity to environmental perturbations associated with an atomic reference frequency is transferred to the variable frequency oscillator.
Hydrogen, cesium and rubidium frequency standards have been used to provide atomic controlled oscillators in which the generated standard frequency is usually 5 megahertz (MHz) or 10 MHz. Such frequency standards have usually employed a quartz crystal oscillator controlled by a physics package and associated electronics in an effort to maintain an accurate and stable standard frequency on a long-term basis. The physics package and associated electronics have been used to slave the quartz crystal oscillator to the frequency of the atomic transition thereby reducing the tendency of the quartz crystal to exhibit drifting due to aging and environmental effects.
Such atomic frequency standards have in the past generally been characterized by means to dissociate the atoms, and means to form the dissociated atoms into narrow beams of atoms with a specific energy level, with said beams being contained within vacuum systems to remove gasses that might interfere with the beam of atoms, and also by means using buffer gasses to contain the atoms. Various electrical and thermal components have been associated with the dissociator and the means to provide the atomic beam. While these various components and elements of the prior atomic frequency standards have been necessary to the operation of the frequency standard, they have introduced long-term and short-term instabilities into the frequency standard. For example, buffer gasses have resulted in collisions with the atoms undergoing energy level transition and have created variations in the frequency of the atomic transition and are a source of long-term instability, particularly in rubidium standards. Dissociators, vacuum pumps, beam focusing and atomic separation means and other such components contribute sources of unreliability and increase the size and the weight and the power requirements of atomic frequency standards.
There has been a continuous and extensive effort in the art to develop reliable atomic frequency standards having short-term and long-term stability in their frequency output, and such improved reliability and reduced size, weight and power requirements as to permit their economical use and transportation. The long and continuous effort to develop improved frequency standards is exemplified by U.S. Pat. Nos. 3,397,310, 3,403,349, 3,536,993, 3,577,069, 3,718,868, 3,967,115, 4,059,813, 4,354,108, 4,425,653, 4,596,962, 4,684,900 and 4,814,728.